PWM (Pulse Width Modulation) control, constant-off-time control and COT (Constant-on-time) control techniques are generally used in step-down regulators.
When COT control is used, the compensation network in the regulator can be eliminated to get a simple structure. Moreover, the regulator with COT control does not need error amplifiers to regulate the output voltage and thus has a better transient response. Because of the above advantages and other advantages, the COT control technique is widely used in step-down regulators.
In COT control, the peak current of the regulator remains constant and the switching frequency varies with the output power to provide a required output voltage. Thus, for applications needing fixed switching frequency, the COT control is no longer applicable.
To solve the problem of variable switching frequency in the COT control, pseudo constant on time (PCOT) control is used in step-down regulators.
In step-down regulators, the duty cycle D is given by Equation (1) as follows:
                    D        =                                            T              on                                      T              sw                                =                                    V              out                                      V              in                                                          (        1        )            , wherein Ton represents the on time of the regulator, Tsw represents the switching period, Vin represents the input voltage of the regulator, and Vout represents the output voltage.
Based on the Equation (1), the switching frequency Fsw of the step-down regulator can be expressed as Equation (2):
                              F          sw                =                              1                          T              sw                                =                                                    V                out                                            V                in                                      ·                          1                              T                on                                                                        (        2        )            
In PCOT control, the on time Ton of the regulator is proportional to the ratio of the output voltage Vout to the input voltage Vin, given by Equation (3) as follows:
                              T          on                =                  n          ⁢                                    V              out                                      V              in                                                          (        3        )            where n is a constant factor.
As can be observed from Equations (2) and (3), in PCOT control, the switching frequency Fsw of the step-down regulator can be expressed as Equation (4):
                              F          sw                =                  1          n                                    (        4        )            
As can be observed from the Equation (4), the switching frequency Fsw is constant as the factor n is constant. That is, in PCOT control, the switching frequency Fsw of the step-down regulator remains constant and does not vary with the output power.
In PCOT control, to ensure the on time Ton of the regulator be proportional to Vout/Vn, one of the prior art step-down regulators comprises an output voltage pin VOUT in the chip to receive the output voltage Vout. The output voltage pin VOUT increases the package size of the regulator chip and also the cost. Besides the output voltage pin VOUT, a frequency setting pin FREQ and/or an external frequency setting resistor RTON may also be needed in some prior art step-down regulator chips, which causes larger package size and higher cost.
FIG. 1 illustrates a prior art step-down regulator 10. As shown in FIG. 1, the step-down regulator 10 comprises a PCOT control circuit 101 and a power stage 102. The PCOT control circuit 101 comprises a constant-on-time (COT) generator 1011, a feedback control circuit 1012 and a logic control circuit 1013.
Referring to FIG. 1, the COT generator 1011 comprises a comparator COMP, a capacitor CTON and a switch K. The first terminals of the capacitor CTON and the switch K are coupled to the non-inverting input terminal of the comparator COMP. The control terminal of the switch K receives a control signal Q provided by the PCOT control circuit 101. The second terminals of the switch K and the capacitor CTON are coupled to ground. The comparator COMP receives the output voltage Vout at its inverting input terminal and generates an on-time signal S1′ at its output terminal. The feedback control circuit 1012 receives a feedback signal Vfb and provides an output signal to the logic control circuit 1013. The logic control circuit 1013 receives the on-time signal S1′ and provides the control signal Q as the output of the PCOT control circuit 101. The power stage 102 receives an input voltage signal Vin and the control signal Q, and provides a switching signal SW.
Referring to FIG. 1, the COT generator 1011 receives the output voltage Vout through the output voltage pin VOUT, and receives the information of the input voltage Vin through the frequency setting pin FREQ and the external frequency setting resistor RTON. Thus, the step-down regulator with the prior art COT generator 1011 comprises the output voltage pin VOUT, the frequency setting pin FREQ and the external frequency setting resistor RTON.
In the step-down regulator 10 of FIG. 1, the package size and the cost are both increased because of the output voltage pin VOUT, the frequency setting pin FREQ and the external frequency setting resistor RTON.